<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <style>
    div {
      width: 200px;
      height: 200px;
      padding: 10px;
      border: 10px solid red;
    }
    div:nth-child(1) {
      /* 传统盒子模型 */
      box-sizing: content-box;
    }
    div:nth-child(2) {
      /* CSS3 盒子模型 */
      box-sizing: border-box;
    }
  </style>
  <body>
    <div></div>
    <div></div>
  </body>
</html>
